Second generation bioethanol is produced from e.g. lignocellulosic fractions of plant biomass that is hydrolysed to free monomeric sugars, such as hexoses and pentoses, for fermentation to ethanol. Lignocellulosic hydrolysates contain high amounts of acetic acid, which is a potent inhibitor of the fermentative capacities of microorganisms used for ethanol production such as yeasts.
Sonderegger et al. (2004, Appl. Environ. Microbiol. 70: 2892-2897) disclose heterologous expression of phosphotransacetylase and acetaldehyde dehydrogenase in xylose-fermenting Saccharomyces cerevisiae strain. In combination with the native phosphoketolase, Sonderegger et al. thereby created a functional phosphoketolase pathway that is capable of net reoxidation of NADH generated by the heterologous expression of a xylose reductase and xylitol dehydrogenase that are used for xylose utilization in the strain.
Guadalupe et al. (2009, Appl. Environ. Microbiol. doi:10.1128/AEM.01772-09) disclose a Saccharomyces cerevisiae strain wherein production of the by-product glycerol is eliminated by disruption of the endogenous NAD-dependent glycerol 3-phosphate dehydrogenase genes (GPD1 and GPD2). Expression of the E. coli mhpF gene, encoding the acetylating NAD-dependent acetaldehyde dehydrogenase restored the ability of the GPD-disrupted strain to grow anaerobically by supplementation with of the medium with acetic acid.
Yu et al. (2010, Bioresour. Technol. 101(11):4157-61. Epub 2010 Feb. 9) disclose Saccharomyces cerevisiae strains metabolically engineered for improved ethanol production from glycerol by simultaneous overexpression of glycerol dehydrogenase (GCY), dihydroxyacetone kinase (DAK) and the glycerol uptake protein (GUP1).
Lee and Dasilva (2006, Metab Eng. 8(1):58-65) disclose the yeast Saccharomyces cerevisiae engineered to produce 1,2-propanediol from glycerol by inter alia introducing expression of the Escherichia coli mgs and gldA genes.
It is an object of the present invention to provide for yeasts that are capable of producing ethanol from acetic acid and glycerol (and hexoses and pentoses), as well as processes wherein these strains are used for the production of ethanol and/or other fermentation products.